The present invention relates to corrosion inhibitors and/or scale control agents for aqueous systems.
Traditionally chromates, dichromates, nitrites, benzoates, silicates, polyphosphates etc. have been used to combat corrosion. However each of these inhibitors suffers from at least one serious disadvantage. Chromates and dichromates are toxic and disposal of them to water courses presents severe ecological problems. Inorganic nitrates require high dose levels to function effectively and they are also subject to bacterial decomposition. This necessitates the use of an expensive organic biocide since chlorine cannot be used as it reacts with nitrite. Benzoates and silicates also require high dose levels and silicates, furthermore, tend to coat metal surfaces with a hydrated silica gel which is often very difficult to remove.
Polyphosphates are effective at relatively low dosage rates but suffer from hydrolysis to phosphate especially at high temperature and at pHs below 7. The phosphate ions liberated on hydrolysis are not effective as corrosion inhibitors, give rise to sludge problems due to the precipitation of calcium orthophosphate and serve as nutrient for biological growth unless chlorine or another biocide is used.
In an attempt to overcome the problems associated with polyphosphate hydrolysis, corrosion inhibitor systems based on the more hydrolytically stable organic phosphorus components have been developed. Examples of this type of phosphorus compound are phosphonocarboxylic acids, e.g. those described in German Patent Specification No. 2,225,645.
Formulations incorporating phosphonic acid or phosphono-carboxylic acid and polyphosphate are well-known for the control of corrosion and scale formation in aqueous cooling systems. Such formulations normally incorporate a non-ferrous corrosion inhibitor e.g. azoles such as benzotriazole or methylene bis-benzotriazole or other azoles known in this art as corrosion passivators or inhibitors.
These known formulations have the disadvantage however that the polyphosphate may hydrolyse to form an orthophosphate which can then precipitate calcium ions present in the cooling water to form calcium phosphate scale or sludge. Such sludges are undesirable and can cause severe problems such as fouling of heat exchangers, blocking of filters and strainers, inefficient operation of cooling tower, reduced effectivity of the corrosion inhibitor formulation and increased pumping costs or reduced flow rates through the system.
This problem is currently alleviated by reducing the pH of the system (usually by addition of acid) to 7.0-7.5 where the calcium phosphate is more soluble. However, this practice increases the corrosion of ferrous metals and does not necessarily stop all scale/sludge formation.
Such corrosion inhibitor formulations are described in German Offen. No. 2,225,645; U.S. Pat. No. 3,837,803; and German Offen. No. 2,337,100.
The phosphono-carboxylic acids are known as seen in German Offen. No. 2,632,774; U.S. Pat. Nos. 3,923,876; 3,959,168; 4,042,324; 4,052,160 and 4,057,511.
The phosphonic acids are described in U.S. Pat. Nos. 3,935,125; 3,992,318 and 4,105,581.
The polyphosphates are described in U.S. Pat. Nos. 3,992,318 and 4,105,581; and in French Pat. No. 2,213,236.
It has now been found that, by introducing hydrolysed polymaleic anhydride into a corrosion inhibitor formulation containing polyphosphate, and a phosphono-carboxylic acid, the problems of calcium phosphate scale/sludge are eliminated and that the addition of acid is no longer required. In fact the addition of hydrolysed polymaleic anhydride to a formulation used in a system treated with acid can still provide better control of sludge/scale formation than the use of acid alone.
Hydrolyzed polymaleic anhydride is described in U.S. Pat. No. 3,963,636. The relevant portions of said U.S. patent are hereby incorporated by reference for the purpose of defining hydrolyzed polymaleic anhydride or a water soluble salt of such a hydrolyzed polymaleic anhydride.